Conventional converter circuits include a multiplicity of controllable power semiconductor switches, which are switched in a known manner in order to switch at least two switching voltage levels. A converter circuit such as this can be connected to an electrical AC voltage power supply system which can be in the form of a three-phase power supply system. Converter circuits such as these are frequently used in industrial installations, with the converter circuits being coupled to the power supply system via a transformer, and with further fields of use and usage options, such as wind power installations, being feasible, of course.
FIG. 1 illustrates a conventional apparatus for operation of a converter circuit as described above. In this apparatus, the converter circuit has a converter unit 2 with a multiplicity of controllable power semiconductor switches. The converter unit is connected via a transformer to a three-phase electrical AC voltage power supply system 1. It should be mentioned that the three-phase electrical AC voltage power supply system 1 with a three-phase power supply system voltage, a three-phase power supply system current and a three-phase transformer inductance voltage is shown in FIG. 1, for the sake of clarity, as a single-phase equivalent circuit with a single-phase power supply system voltage uG, a single-phase power supply system current iG and a single-phase transformer inductance voltage uLT. As is shown in FIG. 1, the apparatus for operation of the converter circuit has a regulating apparatus 4, which is used to produce a regulating signal SR and is connected via a control circuit 3 for forming a control signal SA to the controllable power semiconductor switches. FIG. 2 shows a conventional regulating apparatus 4. As is shown in FIG. 2, the regulating apparatus 4 includes an addition unit 9 for formation of the regulating signal SR by addition of a regulator voltage uR, a transformer inductance voltage uLT which is formed from a power supply system current iG, and a power supply system voltage uG. The transformer inductance voltage uLT is formed from the power supply system current iG and a transformer inductance LT by means of a calculation unit 10. The regulating apparatus 4 has a regulator unit 5 for formation of the regulator voltage uR by regulating the filtered power supply system current iG at a power supply system current nominal value iGref.
The conventional apparatus mentioned above for operation of the converter circuit and the method relating thereto make it possible to regulate the power supply system current iG, but oscillations above the fundamental frequency can occur in the power supply system voltage uG in an electrical AC voltage power supply system 1. However, these oscillations are highly undesirable and cannot be damped, and therefore cannot be reduced, by means of the apparatus described above for operation of the converter circuit, and the method relating thereto. Oscillations such as these occur, for example, when the power supply system has lightly damped resonance points. FIG. 3 shows a profile such as this, with oscillations, of the power supply system voltage uG according to the above-described conventional arrangement.